(a) Technical Field
The present disclosure relates to a method for shutting down a fuel cell system of a vehicle, which can effectively remove water from the fuel cell system when it is shut down at predetermined temperature or lower.
(b) Background Art
A fuel cell system applied to a hydrogen fuel cell vehicle as an environmental friendly vehicle comprises a fuel cell stack for generating electricity by an electrochemical reaction of reactant gases, a hydrogen supply system for supplying hydrogen as a fuel to the fuel cell stack, an oxygen (air) supply system for supplying oxygen-containing air as an oxidant required for the electrochemical reaction in the fuel cell stack, a thermal management system (TMS) for removing reaction heat from the fuel cell stack to the outside of the fuel cell system, controlling operation temperature of the fuel cell stack, and performing water management function, and a system controller for controlling the overall operation of the fuel cell system.
FIG. 1 is a schematic diagram of a typical fuel cell system.
As shown in FIG. 1, the fuel cell system includes a hydrogen supply system 110 and an air supply system 120. The hydrogen supply system 110 includes a hydrogen tank 111, a hydrogen supply valve 112, high-pressure and low-pressure regulators 113 and 114, and a hydrogen recirculation system 116. The air supply system 120 includes an air blower 122 and a humidifier 123.
In the hydrogen supply system 110, the high-pressure hydrogen of the hydrogen tank 111 sequentially passes through the high-pressure and low-pressure regulators 113 and 114 such that the hydrogen pressure is regulated, and the pressure-regulated hydrogen is supplied to an anode (“fuel electrode” or “hydrogen electrode”) of a fuel cell stack 200. In the hydrogen recirculation system 116, an ejector 115 and a hydrogen recirculation blower 117 are provided at an outlet of the anode of the fuel cell stack 200 to recirculate unreacted hydrogen of the anode remaining after the reaction to the anode, thus recycling the hydrogen.
In the air supply system 120, the dry air supplied by the air blower 122 passes through the humidifier 123 to be humidified by absorbing water from the exhaust gas discharged from a cathode (“air electrode” or “oxygen electrode”) of the fuel cell stack 200 and is then supplied to the cathode of the fuel cell stack 200.
A hydrogen purge valve 131 is provided at an anode outlet of the fuel cell stack 200 to discharge foreign substances such as nitrogen and water accumulated in the anode, and the water produced in the fuel cell stack is collected in a water trap 132 and then discharged.
The fuel cell system having the above-described configuration generates electricity by the electrochemical reaction between hydrogen as a fuel and air as an oxidant and discharges heat and water as by-products.
Meanwhile, one of the most difficult problems of the fuel cell vehicle is to improve cold startability. When the fuel cell system is exposed to a temperature below the freezing point of water and kept for a long time (cold soaking), the water present in the fuel cell stack and various components of the fuel cell system such as valves freezes, thus making it difficult to start the vehicle.
Especially, when the freezing occurs in the fuel cell stack, the respective flow fields and gas diffusion layers are clogged, and thus the reactant gases are not smoothly supplied to the fuel cell stack. Therefore, a normal electrochemical reaction does not take place, and the voltage of the fuel cell stack is not kept constant, which makes it difficult to ensure cold startability.
In order to improve the cold startability of the fuel cell system, a variety of techniques such as a method of rapidly thawing the frozen water in the fuel cell stack by heating coolant circulating through the fuel cell stack have been proposed. However, it is necessary to manage the status of the fuel cell system during shutdown together with the rapid thawing of the frozen water in the fuel cell stack.
For example, the water present in the fuel cell stack may be removed in advance when the fuel cell system is shut down at a temperature below the freezing point. In the case where the fuel cell system is shut down under the condition that the ambient temperature is below the freezing point, it is necessary to remove the water produced in the fuel cell stack to ensure stability during the next start-up. If the fuel cell system is shut down while the product water is not removed from the fuel cell stack, the surface of the inside of the fuel cell stack freezes, thus making it difficult to start the vehicle.
To solve the above-described problems, there have been proposed a variety of methods of improving the cold startability by removing the water present in the fuel cell stack such as the inside of a membrane electrode assembly by flowing the fuel (hydrogen) or oxidant (air) through the fuel cell stack when a shutdown is detected at a temperature below the freezing point.
For example, U.S. Pat. No. 6,479,177 discloses a method of removing water remaining in a fuel cell stack by flowing dry gas through the fuel cell stack during shutdown when the temperature is below the freezing point. U.S. Pat. No. 6,887,598 discloses a method of removing water from a fuel cell stack by discontinuing reactant humidification before shutdown of the fuel cell stack and increasing the amount of air supplied. U.S. Pat. No. 7,270,903 discloses a method of removing water from a fuel cell stack by measuring the temperature in the vicinity of a fuel cell vehicle via a thermostat when the fuel cell stack has been shut down for a long time and, when the temperature is below zero, flowing air through the fuel cell stack. U.S. Pat. No. 7,344,795 discloses a method of removing water from a fuel cell stack by supplying air bypassed through a humidifier and an intercooler to the fuel cell stack during shutdown. Besides, there has been provided a method in which the exhaust gas of the fuel cell stack 200 supplied to the humidifier 123 is cut off to stop the humidification of the humidifier 123, the dry air passed through the humidifier 123 by the air blower 122 is supplied to the cathode of the fuel cell stack 200 to remove water from the cathode, and then the fuel cell system is shut down.
However, the above-described methods have the problem that the water remaining in the fuel cell stack is not completely removed, and thus it is impossible to ensure stable startability during the next cold start. Especially, it is difficult to completely remove water from the fuel cell stack only by flowing dry air through the fuel cell stack, and further it is not easy to remove water from the electrolyte membrane as well as from the flow fields such as the cathode channel.
Although it is suitable to supply heated gas to the fuel cell stack, the amount of heated gas is small if there is no heater. Moreover, although the heat generated from the air compressor may be used instead of using the heater, the amount of heat generated is too small to be used in an atmospheric pressure system which employs a blower.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.